The invention relates to an adhesive tape which has been given a pressure-sensitive adhesive coating, having an elastomeric backing layer, a single- or double-sided energy-elastic interlayer and a pressure-sensitive adhesive layer on one or both sides.
The use of interlayers between the actual backing layer and the pressure-sensitive adhesive coating is known. One of the purposes of these interlayers is to prevent or lessen the migration of constituents from the backing layer into the pressure-sensitive adhesive. Migration of this kind may lead to structural alterations in the pressure-sensitive adhesive layer and/or to deposits on the surface of the pressure-sensitive adhesive, and hence to a reduction in the adhesion. A further function of the interlayers is to smooth out uneven backing surfaces.
In the case of the bonded adhesive tape, interlayersxe2x80x94especially if they are energy-elastically deformablexe2x80x94may have the function of absorbing stress peaks greater than the average force acting on the bond area and of distributing such peak stresses over the area of the bond in order thereby to counter fracture of the bond.
For example, it is known for this application to use thin layers of polymers which are in an energy-elastic state thermodynamically owing to their high glass transition temperature and/or their partial crystallinity. Polyamides in particular meet the requirements that are placed on an interlayer.
A disadvantage of the interlayers consisting of polyamides, however, is that, while they often fulfil the barrier, smoothing and force-absorbing function, at the same time their adhesion to the underlying backing is inadequate. At relatively high stress levels, especially in the case of stresses in which peel forces become active, there is a possibility that the adhesive tape might suffer delamination, instances of which may lead to complete failure of the bond.
The object of the invention, therefore, was to provide an adhesive tape which, with an elastomeric backing layer, has an energy-elastic interlayer anchored securely on the backing layer.
To achieve this object the invention proposes an adhesive tape which has been given a single- or double-sided pressure-sensitive adhesive treatment and has an elastomeric backing layer, the polymeric framework of the backing layer consisting essentially of natural rubber or of a mixture of natural rubber with at least one styrene-butadiene rubber.
The styrene-butadiene rubber of the backing layer can be a random copolymer having a styrene content of about 23% by weight. The natural rubber or mixture of natural rubber with styrene-butadiene rubber that is used for the backing layer is crosslinkable by means of electron beams.
In accordance with the invention the backing layer comprises an electron beam-crosslinkable, polyfunctional crosslinker, preferably oligoacrylate or oligovinyl ether, which is partially soluble in the polymeric framework and whose physical forces of interaction with the polymeric framework are so low that it tends towards diffusion to the surface of the backing layer where, together with its fraction which is not dissolved in the polymeric framework, it forms a thin film which contacts the entire area of the polyamide interlayer.
In that case the electron beam-crosslinkable polyfunctional crosslinker is at least partially soluble in the polyamide and is therefore able to diffuse into it. Irradiation with electrons crosslinks the pure framework of the backing, the framework of the backing together with the crosslinker, the pure crosslinker dispersed in the backing, the crosslinker present like a film on the surface of the backing, and those fractions of the crosslinker that have diffused into the polyamide interlayer, this crosslinking possibly taking place with attachment to the polyamide, so that an indissoluble, predominantly covalent attachment of the interlayer to the backing is achieved.
It is advantageous if the combined product composed of backing, crosslinker film and polyamide is irradiated with electrons before the latter has diffused to that side of the polyamide which is opposite the rubber backing.
The combined product can be irradiated, for example, with an electron accelerator of the scanner type at a radiation dose of from 10 to 200 kGy, preferably from 50 to 100 kGy and, with particular preference, 80 kGy. The required acceleration voltage depends on the thickness of the combined product that is to be crosslinked. To achieve sufficient homogeneity of crosslinking, the combined product can be irradiated from both sides.
The crosslinked combined product composed of backing, crosslinker film and polyamide interlayer preferably carries, on the interlayer, a polyacrylate-based pressure-sensitive adhesive, which may or may not be crosslinked.
The pressure-sensitive adhesive can be crosslinked separately on an auxiliary support or in combination with interlayer and backing by means of a chemical reaction, via an inherent mechanism and/or physically by irradiation with electrons in the course of the crosslinking of the backing with the interlayer.
The crosslinker which is supplied to the backing, diffuses onto its surface as a soluble fraction and is deposited there, inter alia, as an insoluble fraction, and which, following partial diffusion into the adjacent polyamide interlayer, brings about anchorage of the interlayer to the backing following its crosslinking with electron beams, is a polyfunctionalxe2x80x94especially di-, tri- or tetrafunctionalxe2x80x94oligoacrylate or an oligovinyl ether having terminal, electron beam-crosslinkable vinyl groups. The base compound carrying the vinyl groups may, for example, be an ethoxylated or non-ethoxylated polyol, an oligoether, or an oligourethane having terminal OH groups.
In accordance with the invention the crosslinker can be admixed alone or in a blend with one or more other crosslinkers of the same or different functionality to the backing formulation.
For example, a bifunctional urethane acrylate having an average molecular weight of 5000 or a tetrafunctional oligoether acrylate having an average molecular weight of 1000 or a tetrafunctional oligoether acrylate, such as ethoxylated trimethylolpropane triacrylate, can be employed per se. It is also possible, however, to blend the bifunctional urethane acrylate or the tetrafunctional oligoether acrylate in equal or unequal parts with the trifunctional ethoxylated trimethylolpropane triacrylate.
Preferably, the crosslinker or the mixture of different crosslinkers is added to the backing formulation in an amount of from 5 to 20% by weight, especially 7% by weight.
To increase the durability of the backing it can be admixed with customary anti-ageing agents, which, depending on the particular application, may originate from the class of the discolouring or non-discolouring anti-ageing agents, in particular in the range from 0 to 10 phr, and also with known light stabilizers or ozone protectants. Blending with plasticizers is also possible.
In order establish specifically the desired properties of the backing it is possible if desired to use fillers. For instance, the natural rubber or the mixture of natural rubber and styrene-butadiene rubber can be admixed with reinforcing, semi-reinforcing or non-reinforcing carbon blacks, in particular at from 0 to 50 phr, with zinc oxide, in particular at from 0 to 50 phr, and/or with other fillers, such as silica, silicates or chalk. Fillers other than those mentioned can also be used. It is possible in addition to add resins from the class of the phenolic resins and/or hydrocarbon resins, in the range, in particular, of from 0 to 50 phr. Depending on the intended application of the adhesive tape, all of these additives referred to can be employed either alone or in any desired combination with one another to produce the backing, in order to obtain optimum tailoring to the utility. Through the use of these additives it is also readily possible to provide the black coloration of the backing which is generally required by the industry.
The backing layer has a thickness of from 400 to 3000 xcexcm, in particular from 500 to 1400 xcexcm.
The backing mixture is preferably prepared in an internal mixer of the kind typically used for elastomer compounding. Processing takes place preferably without solvent. The electron beam-crosslinkable polyfunctional crosslinker and the desired fillers may be added to the natural rubber or to the mixture of natural rubber with styrene-butadiene rubber at the same time. The resulting mixture can subsequently be extruded or calendered to the desired thickness on customary commercial machines. This gives a backing layer thickness that is within the ranges set out above.
In a preferred embodiment the interlayer consists of a polyamide or of a mixture of polyamides in which polyfunctional oligoacrylates are partly soluble; for example, it consists of an amorphous ternary copolyamide having alicyclic units, which copolyamide may have been blended with minor amounts of a partially crystalline polyaminoamide. In that case it is possible to use a polyamide which includes alicyclic segments or a mixture of polyamides whose excess component includes alicyclic segments.
In this context, the interlayer can be produced in a separate operation from the solution or without solvent, by extrusion with or without an auxiliary support, specifically in a thickness of from 5 to 40 xcexcm, especially 8 xcexcm. If an auxiliary support is used, the interlayer can be peeled from this support again with no restrictions.
The pressure-sensitive adhesive consists preferably of copolymers of 2-ethylhexyl acrylate, butyl acrylate, methyl acrylate and acrylic acid, preferably with a composition of from 50 to 70% by weight 2-ethylhexyl acrylate and/or butyl acrylate, from 20 to 40% by weight methyl acrylate, and from 0 to 10% by weight acrylic acid.
The addition of further, known components is possible in order to obtain certain properties, especially the addition of further comonomers, crosslinkers, tackifier resins and anti-ageing agents. The pressure-sensitive adhesive can be processed as a solution, dispersion or melt. It can be applied directly or by the transfer technique to the interlayer. Chemical crosslinking of the pressure-sensitive adhesive with crosslinking agents and/or physical crosslinking with electron beams is advantageous. The applied weight of the pressure-sensitive adhesive can likewise be chosen arbitrarily in dependence on the intended use, within the range from 10 to 250 g/m2, preferably from 40 to 150 g/m2. Furthermore, one or both pressure-sensitive adhesive layers may be covered with a release paper, preferably with a double-sidedly anti-adhesively coated masking paper or with a polyolefin-based film. The film can have been antiadhesively treated on both sides.
Also embraced by the concept of the invention are processes for producing an adhesive tape of the invention.
In one preferred production process the backing layer, one or both interlayers and one or both coats of the pressure-sensitive adhesive are brought together by a laminating process.
In a second preferred process, the backing layer is first of all shaped by a calendering process on the ready-formed interlayer, which is supported by an auxiliary support. The other side of the backing layer is then covered, if desired, with a second interlayer, including auxiliary support, or, in the case of an auxiliary support provided on both sides with an interlayer, is joined with the second interlayer on the other side while the laminate is being wound up into a roll. As soon as the crosslinker film on the backing surface has come into contact with the polyamide of the interlayer, the dissolution and diffusion of the crosslinker into the polyamide begins. Experience has shown that the unreacted crosslinker present on the surface of the interlayer is able to diffuse into the pressure-sensitive adhesive and bring about a deterioration in the force of its adhesion to substrates encountered in practice. Likewise, fractions of the crosslinker which have already been crosslinked, on the surface of the interlayer, may reduce the spontaneous adhesion of the pressure-sensitive adhesive on the interlayer. It is therefore advantageous for the laminate composed of backing and interlayer to be irradiated with electrons before the crosslinker has diffused to the interlayer surface opposite the backing. In the next step of the process the auxiliary support or supports is or are peeled off, and, finally, the pressure-sensitive adhesive is applied directly or by the transfer technique to one or both interlayers.
The present invention describes the construction of a self-adhesive tape which exploits the electron beam crosslinkability of a polyfunctional (at least bifunctional) crosslinker which is partially soluble, and therefore diffuses to the surface of the backing, and is partially insoluble, and therefore becomes deposited, inter alia, on the surface of the backing, this crosslinker being in uninterrupted contact with that fraction that has remained in the backing as well as with that fraction which has diffused into the interlayer. The self-adhesive tape backing of the invention consists of natural rubber or of a mixture of natural rubber with a styrene-butadiene rubber, and the interlayer of the invention consists of polyamide which comprises alicyclic segments and in which the polyfunctional crosslinker is soluble. The firm connection of the two layers, brought into contact with one another preferably by dry lamination, is brought about essentially by the electron-beam curing of the polyfunctional crosslinker. The resulting anchorage of the interlayer on the backing ensures that there is no delamination of the adhesive tape, even under high stresses.
The diffusion of the crosslinker to the surface of the backing and further into the polyamide interlayer, this diffusion preceding irradiation with electrons, takes place spontaneously at room temperature and can be accelerated by elevated temperature. The highest level of adhesion of the pressure-sensitive adhesive on the interlayer is obtained when irradiation with electrons is carried out before the crosslinker has diffused to the interlayer surface facing the pressure-sensitive adhesive. The time required by the crosslinker to reach that surface is of course influenced by the composition and thickness of the interlayer, by the structure of the crosslinker, by its solubility in the interlayer and by the temperature. Experience has shown that, with an adhesive tape construction in the preferred embodiment of the invention, there is no adverse effect on the adhesion of the pressure-sensitive adhesive in the above sense until the uncrosslinked combination product of backing and interlayer with free, diffusable crosslinker is stored at a temperature of up to 40xc2x0 C. for more than 4 weeks.